Spatial and seasonal patterns of larval fish assemblages in the southern Gulf of Mexico

BULLETIN OF MARINE S lEN E 62( I) 17-30 19~~

SPATIAL AND SEASONAL PATTERNS OF LARVAL FISHASSEMBLAGES IN THE SOUTHERN GULF OF MEXICO

Laura Sanvicente-Aiiorve Cesar FLores-Cotoand Laura Sanchez- Velasco

ABSTRACT

Larval fish asscmblages and the factor affecting their spatial and seasonal dynamics inIhe oUlhern Gulf f Mexico were studied The ichthyoplankton material came from threeoceanographic cruises during spring (1983) winter (1984) and summer (1987) To determineichthyoplankton assemblages a c1ustcring mcthod bascd on the Bray-Curlis dissimilarityindex was applied TllIcc distincl fish larvae ascmblages wcre idcntified conesponding tolhe physiography of thc area Occurrcnce of the main fish larvae spccies in each one renectsprincipally the spawning stratcgics of adult fish The coastal assemllage ccupied the eastshallow coastal area its characteristic species being closely related to the esLlInline and la-goonal systems comprising mainly Sciaenidae arangidae and Triglide The neritic IS-

semblage occupied the continental shelf and ils chmucteristic species consisted of larvacwhose adults inhabit thc neritic provincc and spawn in thc samc zonc like some specics ofBothidae Cynoglossidae and Bregmlccroticlre The oceanic assemblage occupied deepwater areas and its main components were species whose adults inhabit thc mesopelagicprovince most were myctophid and gonostomatid Ishes Major flctors affecting palial andscasonIi variations of ichthyoplankton ascmblages are proposed to be the nluin circulationpliltem in the southern gulf continental wltuer dischtrges mixing processes and fish spnwningarens and periods

Distribution of fish larvae is rongly affected by spawning seasonality eggproduction variability predation on eggs and larvae larval starvation durationof planktonic life oceanographic features affecting plankton production larvaltransport and competition (Richardson et a1 1980 Lasker 1985) The spawningstrategie of fishe the means of introducing the eggs or larvae to the planktoncommunity and the trategie for ensuring dispersal to new areas are quite nu-merous The different combinations of spawning strategies in fish and the methodsLlsed [0 carry the eggs and larvae to the plankton give diverse but di tinct patternsto plankton groups (Leiby 1986) These life-history strategies of fishes must becompatible with environmental features for species to p rsist Since the numberof strategies meeting these requirements is finite several species may convergeon the same strategy a consequence of th se adaptative convergence is the for-mation of ichthyoplankton assemblages (McGowen 1993)

Studies on fish larval groups shed new light on the interactions between speciesand the environment breeding strategies of fishes spawning seasonality larvaldrift and types of larval associations Kendall (1975) estab]jshed the existence ofdifferent association among the fish larvae off the Middle Atlantic States Rich-ardson and Pearcy (1977) and Richardson et al (l 980) demonstrated persistentdistribution patterns of fish larvae along the Oregon coast Doyle and Ryall (1989)analyzed the spatial patterns of the icbthyoplankton communities off the Irishcoast Moser and Smith (1993) established a positive correlation among larvalfish assemblages and the variations of the California Currentmiddot Richards et al(I993) determined the role of th Loop Current in the Gulf of Mexico on thedisnibution pattern of fish larvae Olivar and Shelton (1993) mentioned the influ-ence of the Benguela Current on larval fish assemblage and Laprise and Pepin(1995) described the interannual and seasonal variation of ichthyoplankton spatial

17

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NelReoc bullbullbull

BULLEn F MAIlINIo S leN E VOL 62 NO I 1998

Figure I Siudy alc and location of occanogr_lphic slations during the eiFFcrenl sampling periodsin the southern Gulf of Mexico

patterns in the coastal environment of Conception Bay Canada In Mexican wa-ters Flore -Coto t al (1988) calTi d out a study on the larval Ii h associationin Campeche Bay and Sanchez-Velasco and Flores-Coto (1994) defined fish lar-vae assemblages of the Yucatan Shelf and in the Mexican Caribbean Sea Theaim of the present research is to characterize the larvaJ fish assemblages in thesouthern Gulf of Mexico to defin their characteristic species and to understandthe factors affecting their spatial and seasonal dynamics Despite the variabilityin sampling design the present analysis represents an approach that categorizeslarval fish assemblages in the southern Gulf of Mexico during the seasons sam-pled Previous studies on ichthyoplankton distribution in the area suggest thatchanges are geographical rather than easonal (Flores-Coto et aI 1988 Flores-Coto and Sanchez-Ramirez ]989 Flores-Coto ~nd Ord6nez-L6pez 1991)

MATERIALS AND METHODS

STUI Y AREA The study atcn comprised neritic and ocelIlic zoncs in the southcrn GulF of Mexicowith iLs northern limiL at parallel 21degN (middotig I) In lhis rcgion the prcvailing circulation is cyclonic( 10li nari Cl 11 1978 cncll and M tTison 1-181) Two principal mechanisms Lhat might accountfor Ihc cyclone observed in Ihe are ha e hecn suggcsted Thcy arc the curl of thc wind sire s (Merrelland Monison 1981) and the currC11lS wl1iltll migrUlc into thc arCa allcr rhcy arc shed from a cyclonicring created by an upwelling north 0 Lh YucHan Peninsula (Cochrane 1969) A numerical modclproposed by Monreal- omez and Saln -dc-Llt6n (1990) suggcsts grltm seasonal variation in the mag-niLude of the curre11l originating from the Loop Currcnt in Fehruary a eyclollie gyre governs rhccirculation of thc clliire Campeche Bay in April currCll velocities deercase mainly ncar the southcoust in MtlY thc cyclonc disappears and tile currcnts dircction is to the west in July currCll velocitiesincrease and a cyclonic ring develops 10 thc cast of thc bay during August and September rhc gyreextends over thc cntire bay and rcmains until February wherc the celller of lhc gyre migraLes LOLheVest

Thrcc metcorological periods havc bccn rClognized in the area nones pcrioe from October toFebrllalmiddotY CharaCleriled by nonh cold winds and slrong occasional storms dry season from Feb-

SANVICENTEmiddotANORVE ET AL GULF OF MEXICO LARVAL FISH ASSEMBLAGES 19

wary 10 May characlerizcd by cast and soulheal winds and low precipitation and middotmiddotrainy seasonwith the Silme wind patterns and high precipitation frolll June 10 Seplember (Yiiez-Arancibia andDay 1982)

The eoasral marine cnvironm nt is greatly influ nced by fluvial discharges mainly in the easternpart where a very wide continental shelf exists S me rivers are panicularly importanr sllch asPapaloapan and Grijalva-Usumacinla The most important lagoonal systems in the area arc Terminosand Carmen-Machona A thcrmo-halinc fronl is formed along the somlleast coast as a consequence

f rhe discharge into the bay from the Grijalva-Usumacintn system (Czilrom ct al 1986 AlalOlTe elaI 1989) Average fluvial discharges from [his system arc 4 X 10123 X 1019 X 10 and 12 X 10ml mo in winter spring summer and fall re peetively tCziumiddotolll et al 1986) The zone located offthe Terminos Lagoon is highly dynamic influenced by coastal hydrographic features On the outershelf oceanic and neritic walers intermix creating a turbulent environment (Villalobos and Zamora1975)

METHODS Zooplankton were collected on lhree oceanographic cmises during three different seasonsspring (from 31 March to 8 April 1983) winter (from 15 [025 February 1984) and summer (from17 July to 5 August 1987) Winter and spring onespond to low rain prccipitation periods whereassummer corresponds to the rainy season A lotal of 141 oceanographic stations were sampled Stationswere located in botl neritic and oceanic waters except dLlring the spring cruise when samples werellIken only in the neritic zonc (Fig I) Samples were collccted in double-obliquc plankton nct towusing a Bongo sampler with a 505 l11-mesh net Sampling depth and [OW time varied from 10 to 200In and 2 10 23 min respectively according 10 the bathymetry Flowll1 ters were placed in each n I

All fish larvae were removed from the samples and identified to the lowesl possible taxon Unidentifiedlarvae 1Ccollnted for Icss thlIl 5 f the lotal abundance which is nOI considered to signilicantlyaffect Ihe ov rail r ults Larval bun lance was normal ized to 10m of ca surfacc

Similarity among staliom was measured l1sing the Bray-Curtis Dissimilarity Index (Bray and CUl1i 1957) a technique which is sufficiently robust for marine data since it is not affected by Illuilipleabsences and givcs more weight to abundant species 111111 to rare ones (Field bull II 1982) Sampleswere grouped by the average sorting method (Legendre and Legendre 1984) to produce dendrogramsPrior to analysis In(x+ I) transformation of the larval abundance data was performed to hOll1ogenilevriances (Taylor 1961) Asscmblagc descriptors the most frequent and abundant species (Pianklt1974) included those fishes showing thc highest values obtained from thc product abundance bypercentage of frequency These descriptors were defined eliminating those species whose productsvalues approached the asymptotic vallIe in the curve oblained by plolling the rank of species versusIhe prodl1cl in a dccreasing oreler

RESULTS

Cluster analysis defined four groups of stations which remained geographicallythe ame during the different seasons Figs 2-4) Species present in three of thesegroups integrated into welJ defined assemblage named coastal neritic andoceanic which are generally persistent in their respective zones The fourthgroup of stations defined a tran itional zone between neritic and oceanic assem-blages

Some taxa had wide distribution in the overall study area Thi is the case ofthe families Gobiidae Engraulidae and Gerridae that were not pecifically iden-tified The first one was found in high abundance in all three assemblages andthe latter two OCCUlTed mainly in the neritic and coastal assemblages (Table 0-The remaining taxa showed affinity to particular hydJOlogical conditions

COASTAL As EMBLAGE The coastal as em biage influenced by fiuvio-lagoonsystems occupied mainly the east shallow coastal area in depths Jess than 36 m(Fig 2-4) The area occupied by thi as emblage varied throughout the seasonsIn winter it comprised the area from the Grijalva-Usumacinta river mouth to theTerminos Lagoon (Fig 2) larvae collected in stations 29 and 30 in the middleof this zone were unidentified In summer the area was bigger extending fromBarra de Dos Bocas to the east of the lagoon including the other fluvio-Iagoonalinfluenced stations (81 and L02) and station 14 (Fig 4) In spring cluster analysisdefined a great coastal assemblage over the inner shelf differences from theadjacent neritic assemblage were mainly due to numerical difference of Engrau-

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095

076

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038

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BULLEoIN OF MARINE SCIENCE YOLo 62 NO I 1998

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I

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Figure 2 A Dissimilarity dendrogram representing group of stations ltlccording their iehthyoplank-Ion composition in lhe southern Gulf of Mexico during winter of 1984 B Gcographical location oftile groups of stalions

lidae Gobiidae Gerreidae and Sardinella anchovia rather than compositionalstructure In this great coastal assemblage the Terminos Lagoon zone formed awell defined subgroup (Fig 3) since it was geographically equivalent to the twoothers seasonal comparisons among this assemblage were made considering this

Djssirrilaritylevel

SANVI ENTE-ANORVE ET AI_ GULF OF MEXICO LARVAL FISH ASSEM8LAGES

A

21

095

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32 27 42 9 20 19 2 3 44 34 11 33 7 15 22 4 14 18 48B 24 25 Stations37 28 8 2 31 38 6 21 10 43 46 45 35 16 36 5 17 48 47 29 30 39

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Figure 3 A DissimilarilY dendrogram represcnting gr ups of slations according their icluhyoplank-[on composilion in the southern Gulf of Mcxico dUling spring or 1983 B Geographical location ofthe groups of stations

ubgroup (no fish larvae were collected In station 40 located to the east of thissubgroup)

Among the taxa characterizing this as emblage (Table 1) the dTUms Cynoscionarenarius and Micropogollias ulldulatlls(fumieri and the hogchoker Trinectes ma-

22

Dissirrilaritylevel

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BULLETIN OF MARINE SCIENCE VOL h NO I 199~

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6710882 T2 6915392 1 32361751132 768lt11076331 10 27 8 84 ~ 41128147202346 16 Stations114 9109 79 7060 fgt51004 34 26 76 30 89 7366106 52 29 28 12 51 97 96 99 87 66 10257 21 37 38 58

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igure 4 A Di~sill1ilarity dendrogram representing groups of stations according Iheir iehthyoplank-ton composilion in the southern Gulf of Mexico during summer of 1987 B Geographical location ofrhe groups of stations

eulalliS showed a nearly exclusive oc unence in this area Larvae of Chloros-eombrus ehlysurus and Opisthonema oglil1l1m showed their greate t abundancenear the river mouths mainly in summer

NERlTl ASSEMBLAGE The neritic assemblage occupied most of the shelf areaexcluding coa tal zones influenced by continental aquatic systems (Figs 2-4)

SA VICENTEmiddotAN RVE ET L G LF OF mXICO LARVAL FIS1-I ASSEMBLAGES 23

Most of the taxa charactelizing this assemblage occurred throughout the yearsome with high frequencies of occurrence (gt50) like the flatfish Symphuruscivitatium the codlet Bregmaceros comori and the families Ophidiidae and Syn-odontidae (Table 1) The Atlantic tlu-eadfin Polydactylus octonemus occurredmainly on the outer shelf off the Terminos Lagoon Some taxa howed a con-spicuous peak of abundance in a given season Haemulidae Mugi curema Mullusspp P octonemus Sardinella onchovia and Syaciwn gunteri in pring Bregma-cems cantor in summer and Ophidiidae in winter

OCEA IC ASSEMBLAG The oceanjc assemblage was localized in deep waterareas apparently extended throughout the platform during swnmer and restrictedto the western part of the study area in winter (Figs 24) Larvae of the Myc-tophidae and Gonostomatidae familie were the most frequent and abundant inthis assemblage Most of the taxa characterizing this assemblage hawed theirgreatest abundance in sununer and had a wide distribution in the oceanic provinceAmong the taxa howing conspicuous peaks of abundance in winter were Ben-thosema suborbitale Hygophum hygomii Lampanyctus spp and Scopelarchidae(Table l) Excepting the gonostomatid Mouroliclls muelleri all taxa had morethan a 50 frequency of occurrence in the two sampled seasons

DIS US ION AND CONCLUSI N

The spatial distribution and the spawning season of the species present in eachassemblage correspond in a general sense with previous records as we showbelow

Engrautids gelTies and gobies are among the most abundant families in thearea the first mainly captured on the inner shelf (Flores-Coto et aI 1988) Larvaeof engrautids have been recorded by Retana-Varela (1989) as widely distributedover the shelf in the southern Gulf of Mexico According to Resendez-Medina(1981) anchovie Anchoa mitchilli A famprotaenia and A hepsetus spawn main-ly in the Terminos Lagoon and on the adjacent littoral band Most of the peciesof Gerreidae are fOWld in f1uvio-lagoon systems and the highe t larval abundancehas been reported in the littoral zone in front of the Terminos Lagoon althoughspawning may occur in the lagoon itself (Flores-Coto 1988) Yaiiez-Arancibiaand Sanchez-Gil (1986) found the gerries Eucinostomus gula and E orgelreusas typical components of the adult fish populations on Campeche Bank Adultsof gobies are quiet euryhaline fishes usually found in very low salinities near thecoa t (Haese and Moore 1977) Spawning of most of these species take placeover the bottom shelf on a number of substrates and parental care occur in manyspecies (Fritzsche 1978)

The wide larval distribution of these families e pecialJy gobies and engraulid(Table 1) might be a consequence of a high larval drift from the shallow areaswhere massive spawning occurs We believe that these dispersion capabilities maybe associated with long-duration larval stages as recorded by Houde and Zastrow( 1993)

COASTALAss -MBLAG Life-cycles of characteristic taxa of this as emblage arerelated to the lagoon-estuarine ystems using them as habitats for larvae juvenilesor adults

Spawning of ChlorosCOl11brus cluysurus and Cynosciol7 arenarius occurs in thecoa tal zone mainly during warm periods (Houde et al ]979 Rivera-Elizalde1988 Flores-Coto and Sanchez-Ramirez 1989) Juveniles of both species inhabitthe Terminos Lagoon and their recruitment to the adult population into the oceanoccurs by migration from the lagoonal system (Tapia-Garda et aI 1988 Tapia-

24 BULLETIN OF MARINE SCIENCE VOL 62 NO 1 1998

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26 BULLETIN F MARINI SCIEN E V L 62 NO I 199R

Garcia 1991 Flores-Coto and Perez-Argudfn 1991) According to Flores-Cotoand Perez-Argudin (1991) Rivera-Elizalde (1988) and Sanchez-Velasco (1989)the spawning of Micropogonias llndulatuslfurnieri Trinectes maculatus and Op-isthonel1la oglinllln occur in the c astaJ band and the larvae penetrate into theestmuies These authors noted hat 0 oglinu11 spawns all year-round showing apeak of abundance in spring and summer whereas M undularusfurnieri and TmaculallS spawn mainly in autumn and winter The pre ence of O oglinum onlyin summer (Table 1) may be due to a drift from the littoral zone where accordingto Sanchez-Velasco (l989) spawning occurs Main spawning seasons of thesespecies are in accordance with our records Regarding the Triglidae family ourfinclings uggest that searobins spawn in zones influenced by estuarine waterwith no apparent seasonality

These results show that larval specie characterizing this assemblage could beconsidered as indicators f c astaJ systems influenced by fluvio-Iagoonal waters

NERITIC ASSEMBLAGEThis assemblage consisted of fish larvae whose adultinhabit the neritic province and spawn in the same zone

Flores-Coto et al (1988) observed that larvae of Bregmaceros cantori are wide-ly distribu ed over the helf mainly in zones of les than 100 m depths and arepresent all year-round with their major abundanc in ummer They also men-tioned that larval tages of Mullus spp as w 1 as some genera of the familySynodontidae (Saurida Synodus and Trachinocephalus) occur widely in the ne-ritic region with Mullus recording its major abundance in winter and spring ourrecord show that winter is the beginning f the pawning period (Table 1) Forspecies of Synodontidae we observed a br ad pawning period Larvae of Sya-dum gUl1teri were recorded by Flores-Coto et al (1991) as the most abundantamong the both ids with highest frequency and abundance in the zone boundedby the 20 and 40 m isobaths during spring and summer Similarly they alsoobserved Symphurus civitatium and Botntmiddot ocellatus among the five most abun-dant flatfish species especially over tbe shelf during the entire year but mainlyin the warm season Tn ur study high winter values (Table 1) suggest greatinterannual fluctuations of abundance of both sp cies Acc rding to Fritzsche(1978) the Atlantic cutlassfish Trichiurus lepturus spawns in the northern gulfin zones deeper than 46 m in the studied area our records inclicate a broadspawning period of this species with highest larva] abundance in winter (Table1) Sardinella anchovia larvae showed greatest abundance in spring (Table 1)this species is known to spawn all year-round in shallow waters especially duringthe warm s ason (Houde et al 1979 Retana- Varela 1989) Larvae of Mugilcurema Etropls crossotus and AIxis spp are mainly captured over the shelf dur-ing the warm sea on (Haese and Moore 1977 Flores-Coto et a1 1988) ourresult confirm these observations Small specim us f Polydactylus octonemusare common in estuaries and shallow waters during spring and summer in thenorthern gulf (Walls 1975) in the tudied ar a postf1exioned larvae were foundon the outer shelf only during the spring (Table I) Larvae of haemulids alsocalled grunts have been registered by Flores-Coto et a1 (1988) over the platformour re ults howed that some species pawn mainly during the spring season(Table 1) In the adult stage the grunt Haemulon plumieri is considered a typicaJcomponent of the demersaJ fish community in Campeche Bay (Yanez-Arancibiaand Sanchez-Gil 1986)

OCEA IC ASSEMBLAGEThe characteristic components were species whoseadults commonly inhabi the mesopelagic province

In general the Myctophidae Diaphus Benthosema NOlolychnus and the Gon-ostomatid Cyclofhone are the most frequen and abundant genera in the oceanic

SANvrCE TEmiddotAmiddot ORVE ET AL GULF OF MEXICO LAIlVAL FISH JSSEMBLAGES 27

province (Flores-Cow and Ordonez-Lopez 1991) However their occurrence inthe neritic zone may be frequent (Badcock 1981) possibly due to a high larvaldrift from the oceanic zone as was observed for Diaphus during the warm season(Table 1) Flores-Coto and Ord6nez-Lopez (1991) mentioned that spawning ofthese taxa occurs throughout the year with no apparent seasonality we foundhowever some seasonal difference for Diaphus and Benthosema (Table L) OurresuJts agree with these authors who mentioned that larval stages of the Myc-tophidae Hygophum hygomii and Lompcmyctus spp and the Sternoptychidae Ster-noptyx sp are mainly distributed in the oceanic zone and are more abundantduring the winter Pineda-L6pez (J 986) mentioned that larvae of Lesticliops offinisare distributed in the oceanic and on the continental slope zones our findingshow that summer is the main spawning season (Table 1) Myctophum nitidulum

and MCiurolicus mueLLeri have a wide distribution on the continental sheJf and inthe oceanic area latter being particularJy abundant during the warm season (Flo-res-Coto and Ordonez-L6pez 1991) Juarez (1975) found that larvae of KatsZl-won us pelomis are abundant dllLing the warm season in both oceanic and neriticprovinces near the Yucatan Peninsula our results show mainly an oceanic distri-bution (Table 1) The larval stage of BregmClceros Cltlanticus in contra t to itcongener B canrori ha an oceanic distribution (Flores-Coto et aI 1988 Tablel) being more abundant in ummer The occurrence of Selar crumenophthCllmusin the oceanic assemblage might be a consequence of larval drift from the neriticzone where according to Flores-Coto and Sanchez-Ramirez (1989) spawningoccurs Flores-Coto et aI (1988) indicated the occurrence of larvae of some Sco-perlachidae species (Scopelarchus micllOelsarsi and S analis) in the oceanic prov-ince during the warm season we found larval specimens only in winter sug-gesting a broad spawning period for pearleyes

TRANSITIO ZONE A fourth group of stations was unique to the continentalslope (Figs 2-4) The most frequent and abundant species in thi zone wereequally represented in both neritic (Syacium gunteri Symphurus civitatium Breg-mCiceros cantori Mugil cephalus Bothus ocellatus) and oceanic assemblages(Diaphus Cyclothone Benlhosema suhorbitale Notolychnus valdiviae Vinci-guerria nimbaria and Bregmaceros atlanticus) The co-occurrence of speciescoming from the adjacent provinces in a large area along the slope indicated theexistence of an important transitionaJ zone between the two assemblages

SEASO AL AND SPATIAL VARIATIONSOF THE ICHTHYOPLAKTONICASSEMBLAGESThe size of the area occupied by the assemblages varied between ampling times(Figs 2-4) The main factors affecting patial variation of ichthyoplankton a -semblages are proposed to be the main circulation pattern in the bay continentalwater discharges mixing processes and fish spawning areas and periods

The area occupied by the coastal assemblage varied depending on the extensionof continental waters into the marine environment Fluvial discharge from therivers Grijalva and Usumacinta r suIts in the formation of a brackish and nutrientrich front extending into the oceanic area (Czitrom et a1 1986 Padilla et al1986 Cananza-Edwards et aI 1993) A cording to Ledenev (1966) hydrologicaJfronts play an es ential role in determining the spatial heterogeneity and dynarrucsof plankton During the winter and spring period when the discharge of therivers is approximately tluee times les than in summer (Czitrom et aI 1986)the area occupied by this assemblage extended nearly to the 18 m isobath (Figs2 3) whereas during the ummer cruise the assemblage occupied a wider areafollowing the coast line (Fig 4) Alth ugh the western limits for this assemblageduring spring and summer were nearly the same tbe cause seemed to be differ-ent the littoral current towards the west during spring (Monreal-G6mez and Salas-

28 BULLETIN OF MARINE SCIEN middotE VOl bull 62 NO I 1998

de-Leon 1990 Monreal-Gomez et al 1992) and the high river discharge duringsummer

The oceanic and neritic assemblages occupied basicaUy their respective prov-inces a a consequence of the spawning area of the constituent species Salinityand temperature of these provinces do not show large easonal fluctuations (Pa-diUa et aI 1986) Pineda-L6pez (1986) detected a rising low alinity laquo3460)and low temperature water mass in th west oceanic zone during the winter of1984 High zooplankton biomass and fi h larvae abundance recorded in this areaduring this period might be a con equence of this nutrient rich cold water (pineda-L6pez 1986 Sanvicente-Anorve ]990) generating an apparent restriction of theoceanic assemblage to the we t (Fig 2) In this season spatial ctistribution of thiassemblage overlaps the center of the 250-350 km ctiameter gyre governing thebay (Monreal-G6mez and Salas-de-Le6n 1990)

The transitional zone showed high spatial dynamism throughout the year re-flecting the importance of mixing processes between neritic and oceanic waters(Villalobos and Zamora 1975) and the great dispersion capability of orne peciefrom the adjacent assemblages During summer this zone is over the shelf andnearly overLaps the center of the newly formed cyclonic gyre (100-150 km cti-ameter) in the eastern gulf (Monreal-G6mez and Salas-de-Le6n 1990) Th zoneis also very evident over the outh and west slop s (Fig 4) where high currentvelocities have been detected by the same authors

Despite the variability in sampling design spatial and temporal sampling scaleswere sufficient to identify the spatial extent or temporal persistence of the assem-blages in the outhem Gulf of Mexico According to Njhoul and Djenidi (1991)the sy tern dynamic should be explained by processes on the same range of timeand length cales Physical and biological time scales characterizing these assem-blages are of the same order of magnitude Thresher and Brother (1989) indicatedthat the biological time scales for fish are on the order of weeks to months thetime needed for the pelagic larvae prior to recruitment to the juvenile populationPhysical factors affecting the main circulation pattern in the southern Gulf ofMexico are on the order of months (Czitrom et a1 1986 Momeal-G6mez andSalas-de-Leon 1990) Spatial scales of these assemblages and of the main cir-culation pattern are on the order of hundreds of kilometers

In general the assemblages defined might be considered indicators for the dif-ferent environments which they inhabit Species belonging to each assemblageshow similarities concerning their reproduction habits or affinity to a particularhydrographic condition Species of the coastal assemblage show a strong eco-physiological dependence upon the lagoon-estuarine systems The members of thecoastal and neritic assemblages are mainly planktivores related to high primaryproduction ystems wherea in the oceanic zone where primary production islower it seems that high larv~l di persion is one of the factors controlling theassemblage structure

ACKNOWLEDGME TS

This research was funded by CONACyT conlract PC NCCNA 031676 frOm Melltico W thank FZavala for his field and technical lt1sislance B van Tuss nbroek for support wilh the English telltland thc anonymous rcviewcrs for lhcir comments

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DATE A -fTED Fcbruary 9 1996

ADDRESS Ulliversidcul Nucif)JwlllrOIl(JlIlli de Mexico fllstiwro de Ciellcumiddot del Mar y Linmolog(lIAparrado POImiddotlal 70-305 04510 Mexico DF Mexico Ellail le(IervidorwUlIIlllx